Jason came to me a few years ago looking for a way to track the popularity and correctness of comments on his site. I developed a solution that was great for him and his users and it has lasted over 3 years now without modification.
The fine folks at Poker Affiliate Solutions help their customers build websites that show various poker related offers, handle user management and process affiliate payouts. They wanted to add another tool to their arsenal and asked me to integrate their API and existing templating language into WordPress. After a lot of work, we arrived at a solution that worked exactly as they wanted.
Sometimes a post or page is updated frequently for some reason. Perhaps it contains daily deals, site notifications or other things like that. The update subscription plugin I built allows individual users to subscribe to updates on a piece of content, confirm their subscription and receive notifications every time the content is updated. The users can also unsubscribe from the notifications at any time.